* add modes and documents for vehicle and pedestrian detection

* remove needless output images

* add class names

* add Configure for training

* fix table in READ.md

* compress the images in the demo/output

* change the score_threshold for vehicle inference

* change Yolo v3 to YOLOv3

This reverts commit 0be3697c.

* add class aware sampling strategy
* remove redundancy code

This reverts commit 36a2e65b.

* Revert "change PyReader to from_generator (#3490)"

This reverts commit d331b799.

* Update data_reader.py

* Update data_reader.py

* Update utility.py

This reverts commit 04bfe0a8.

This reverts commit b591cc13.

* refine assert info in roidb_source.py

* refine image_dir assert

* fix fluid.data and fluid.io.DataLoader

* add logger in utils/utility.py

* Add face detection configs and main code.
* Integration multi-scale evaluation.

* unify reader to dataloader

* add check_version

* change PyReader to from_generator

* fluid.layers.data->fluid.data

* fix_bug

* add_multiprocess_train_to_image_classification

* Document mixed precision training flags

* Clarify GPU is required for mixed precision training

* refine build strategy for the model optimize test=develop

* fix according develop test=develop

* add CropImageWithDataAnchorSampling
* Refine bbox sampling code
* Update comments
* fix import and variable problem

* Add backbone and architecture for FaceBoxes and Blazeface.
* Add dataloader for WIDERFace dataset.

Update ppdet/utils/checkpoint.py to skip the var shape check when the var is not initialized. (#3452)